You will first need to change the boot code from CE to Linux, using the CE programming procedures. Once that is complete, you can reload the Linux files as described in the Linux manual.
Take a look at this topic for an example of how to do this.
Important! Make sure you load the correct Linux boot code onto the BitsyX (NOT the Bitsy code), or you'll end up with a paperweight. If the boot code gets lost, you will have to return the system to the factory for reprogramming.
After you have successfully installed the bootloader, install Linux according to this topic.
Finally after you have linux installed you can install Opie according to this topic.